1. Field of the Invention
The invention relates to a hydrogen permeation membrane in the form of a selective hydrogen-permeable metal coating less than 100 microns thick, specifically 10 microns to 30 microns thick, consisting of a material with a high hydrogen permeation coefficient, specifically on the basis of palladium, with an adjacent gas-permeable support structure.
2. Description of the Prior Art
The high permeability of hydrogen through suitable membranes is used in many ways in industry. The membranes thereby act as a separating wall between two spaces, but one which is hydrogen permeable to the greatest possible extent is very desirable.
The first example which can be cited is the removal of impurities from hydrogen gas: the hydrogen current to be decontaminated is trained along a separating wall consisting of Pd or Pd-Ag. By permeation, high-purity hydrogen is recovered on the secondary side, while the impurities remain on the primary side. For a high hydrogen-permeation flow-in addition to other operating and material parameters-the wall thickness of the separating membrane must be as low as possible.
Another example is the optionally selective or nonselective separation described in DE-OS No. 3 121 125 of certain isotopes of hydrogen, e.g. tritium, from a mixture of isotopes of hydrogen: the hydrogen isotopes permeate a separating membrane, interact with a substance present on the secondary side, and are removed with this latter substance. The choice of the substance makes it possible to either remove all the hydrogen isotopes or to selectively remove only a given isotope, e.g. tritium.
The separating walls which are used in the processes described above and in similar processes must have a high permeability for hydrogen. Materials which fall into this category include metallic substances such as Nb, Ta, V, Pd and Zr, but also certain glasses and plastics. The prevailing ambient conditions, however, can lead to significant limitations. These limitations, for the metallic substances, include embrittlement from the formation of hydrides and the severe reduction of the hydrogen permeation coefficients as a result of the formation of blocking cover coatings on the surface of the membrane, e.g. in the form of metal oxides. For this reason, preferance is given to the use of palladium and its alloys, which are largely resistant both to the formation of hydrides and also to surface oxidation. But with palladium, the principal disadvantages are its high price and its limited availability. For this reason, membranes which are as thin as possible are desirable. On account of the required mechanical stability, the lower limit of the feasible wall thicknesses for thin-walled tubes is about 70 microns, even if reinforcement elements are provided inside, as spiral springs (DE-PS No. 1 467 079).
To further decrease the wall thickness for the Pd, porous carriers have been suggested, e.g. made of sintered metal, on which a thin Pd coating of up to 12 microns thick is applied, e.g. by vaporization (U.S. Pat. Nos. 2,824,620 and 3,241,298). But with porous carriers which are made of granular material such as metal particles, one disadvantage is their required wall thickness of approximately 500 microns to 1000 microns for the desired strength. This leads to a significant reduction of the effective permeation velocity through the overall separation wall.
While the effective permeation velocity of the hydrogen through a free-standing Pd tube is determined only by the permeation step through the metal, with the combination of a porous carrier 500 microns thick and a Pd coating 5 microns thick, the diffusion velocity in the porous carrier becomes the dominant factor for the effective permeation velocity.
Since the permeation surface required for the achievement of a given permeation flow, and thus the Pd requirement, is determined by these factors, the advantage of the low thickness of the Pd coating can be utilized only very incompletely.